1. Field of the Disclosure
The present application relates to image sensors suitable for sensing radiation in deep UV (DUV), vacuum UV (VUV), and extreme UV (EUV) wavelength, and to methods for making such image sensors. Some embodiments of the sensors are suitable for sensing electrons and other charged particles. All of the sensors are suitable for use in photomask, reticle, or wafer inspection systems.
2. Related Art
The integrated circuit industry requires inspection tools with increasingly higher resolution to resolve ever smaller features of integrated circuits, photomasks, reticles, solar cells, charge coupled devices etc., as well as detect defects whose sizes are of the order of, or smaller than, those feature sizes.
Inspection systems operating at short wavelengths, e.g. wavelengths shorter than about 250 nm, can provide such resolution in many cases. In other cases, electrons or other charged particles, such as helium (He) nuclei (i.e. alpha particles) may be used. Specifically, for photomask or reticle inspection, it is desirable to inspect using a wavelength identical, or close, to the wavelength that will be used for lithography, i.e. close to 193.4 nm for current generation lithography and close to 13.5 nm for future EUV lithography, as the phase-shifts of the inspection light caused by the patterns will identical or very similar to those caused during lithography. For inspecting semiconductor patterned wafers, inspection systems operating over a relatively broad range of wavelengths, such as a wavelength range that includes wavelengths in the near UV, DUV, and/or VUV ranges, can be advantageous because a broad range of wavelengths can reduce the sensitivity to small changes in layer thicknesses or pattern dimensions that can cause large changes in reflectivity at an individual wavelength.
In order to detect small defects or particles on photomasks, reticles, and semiconductor wafers, high signal-to-noise ratios are required. High photon or particle flux densities are required to ensure high signal-to-noise ratios when inspecting at high speed because statistical fluctuations in the numbers of photons detected (Poisson noise) is a fundamental limit on the signal-to-noise ratio. In many cases, approximately 100,000 or more photons per pixel are needed. Because inspection systems are typically in use 24 hours per day with only short stoppages, the detectors are exposed to large doses of radiation after only a few months of operation.
A photon with a vacuum wavelength of 250 nm has energy of approximately 5 eV. The bandgap of silicon dioxide is about 10 eV. Although it would appear that such wavelength photons cannot be absorbed by silicon dioxide, silicon dioxide as grown on a silicon surface must have some dangling bonds at the interface with the silicon because the silicon dioxide structure cannot perfectly match that of the silicon crystal. Furthermore, because the single dioxide is amorphous, there are likely also some dangling bonds within the material. In practice, there will be a non-negligible density of defects and impurities within the oxide, as well as at the interface to underlying semiconductor, that can absorb photons with deep UV wavelengths, particularly those shorter than about 250 nm in wavelength. Furthermore, under high radiation flux density, two high-energy photons may arrive near the same location within a very short time interval (nanoseconds or picoseconds), which can lead to electrons being excited to the conduction band of the silicon dioxide by two absorption events in rapid succession or by two-photon absorption. EUV photons have very high energies (13.5 nm in wavelength corresponds to photon energy close to 92 eV) and are capable of breaking silicon-oxygen bonds as well as strongly interacting with defects and contaminants in the oxide. Electron and charged-particle detectors typically have to detect electrons or charged particles with energies of a few hundred eV or higher. Energies greater than 10 eV can readily break silicon-oxygen bonds.
As indicated above, high-energy photons and particles can break bonds and ionize atoms in a silicon dioxide layer. Because silicon dioxide is a good insulator, free electrons created in the silicon dioxide may have lifetimes of ms or longer before recombining. Some of these electrons may migrate into the semiconductor material. These electrons create electric fields within the silicon dioxide and between the silicon dioxide and semiconductor. These electric fields can cause electrons created in the semiconductor by absorption of photons to migrate to the surface of the semiconductor and recombine, thereby resulting in lost signal and reduced detector quantum efficiency. Near continuous use of the instrument means that there may be little, or no, time for recovery of the detector, as new free charges are created as fast as, or faster than, they can recombine.
High-energy particles and photons can also cause irreversible changes to the silicon dioxide. Such changes can include reconfiguration of the bonding of atoms or migration of small atoms within the silicon dioxide. At normal operating temperatures of the detector, which are typically in a range from around room temperature to about 50° C., these changes will not recover. In particular, it is known that conventional silicon photodiodes used as EUV detectors degrade in efficiency with use.
The silicon dioxide layer on the surface of semiconductor detectors significantly reduces the efficiency of those detectors for low-energy (less than about 2 kV) electrons. Some low-energy electrons are absorbed by the silicon dioxide, thereby causing the silicon dioxide to charge up and deflect subsequent arriving electrons. Because a native oxide will always form on an exposed silicon surface, silicon detectors necessarily must have some oxide on their surface. Growing or depositing an alternative dielectric material (instead of the oxide) on the surface of the semiconductor results in a much higher density of defect states at the semiconductor to silicon dioxide interface. These defects reduce the quantum efficiency of the detector, especially for photons or charged particles absorbed close to the surface of the semiconductor.
An additional cause of degradation of EUV sensors is that, in a EUV system, a thin layer of carbon builds up over time on any surface exposed to EUV radiation, including the surface of the image sensors and optical elements. This carbon layer, as it becomes thicker, absorbs EUV radiation and reduces the sensitivity of the sensor, as well as reducing the reflectivity of optical elements in the light path. In a EUV system, all surfaces exposed to EUV are periodically cleaned to remove the carbon. This cleaning is usually performed with activated hydrogen (a mixture of atomic hydrogen and hydrogen radicals), which is very effective at removing carbon. However hydrogen radicals affect the oxide on the surface of silicon detectors and can also cause degradation of the performance of those sensors.
Diode detectors suitable for detecting EUV and/or electrons are known in the art. Exemplary diode detectors are described in U.S. Pat. No. 8,138,485 issued to Nihtianov on Mar. 20, 2012, U.S. Pat. No. 7,586,108 issued to Nihtianov on Sep. 8, 2009, US Published Application 2012/0268722 published on Oct. 25, 2012 (filed by Nihtianov), and US Published Application 2011/0169116 published on Jul. 14, 2011 (filed by Nanver). These diode detectors include a thin (1 nm to 20 nm) layer of boron directly on the silicon surface. US Published Application 2011/0169116 further describes an open mesh of a metallic conductor on the surface of such a detector.
These prior-art detectors have contacts formed on the top (light or electron-incident) surface. A disadvantage of having contacts and conductors formed on the illuminated surface is that it is not possible to create a detector with a large number (thousands or millions) of detector elements (pixels) while maintaining high detector efficiency. Each detector element requires multiple control signals, which are typically shared with other detector elements. For full-well capacities of 100,000 electrons or more, detector element dimensions may typically be in the range of about 10 μm to 20 μm. It is not possible to make hundreds or thousands of interconnects that connect these control signals to one another and to drive circuits without covering a substantial fraction of the area of the surface. Because DUV, VUV, and EUV photons and low-energy particles will not penetrate through layers of conductors such as metals and poly-silicon, the area covered by these conductors will have low, or no, sensitivity.
Therefore, a need arises for an image sensor capable of detecting high-energy photons or charged particles yet overcoming the above disadvantages.